In the field of semiconductor processes, the design rule is going into an age of 100 nm and the production form is on a transition from a mass production with a few models representative of a DRAM into a small-lot production with a variety of models such as a SOC (Silicon on chip). This results in an increase of a number of processes, and an improvement in an yield for each process must be essential, which makes more important an inspection for a defect possibly occurring in each process. The present invention relates to a substrate inspection method for inspecting a substrate such as a wafer after respective processes in the semiconductor process by using an electron beam, a substrate inspection apparatus to be used therefor and an electron beam apparatus for the inspection apparatus, and a device manufacturing method using the same method and apparatuses.
In conjunction with a high integration of semiconductor device and a micro-fabrication of pattern thereof, an inspection apparatus with higher resolution and throughput has been desired. In order to inspect a wafer substrate of 100 nm design rule for any defects, a resolution in size equal to or finer than 100 nm is required, and the increased number of processes resulting from a high integration of the device has called for an increase in the amount of inspection, which consequently requires higher throughput. In addition, as a multilayer fabrication of the device has been progressed, the apparatus has been further required to have a function for detecting a contact failure in a via for interconnecting a wiring between layers (i.e., an electrical defect). In the current trend, an inspection apparatus of optical method has been typically used, but it is expected that an inspection apparatus using an electron beam may soon be of mainstream, substituting for the inspection apparatus of optical method in the viewpoint of resolution and of inspection for contact malfunction. The inspection apparatus of electron beam method, however, has a weak point in that the inspection apparatus of electron beam method is inferior to the inspection apparatus of optical method in the throughput.
Accordingly, an apparatus having higher resolution and throughput and being capable of detecting the electrical defects has been desired to be developed. It has been known that the resolution in the inspection apparatus of optical method is limited to ½ of the wavelength of the light to be used, and it is about 0.2 μm for an exemplary case of a visible light having put to practical use.
On the other hand, in the method using an electron beam, typically a scanning electron beam method (SEM method) has been put to practice, wherein the resolution thereof is 0.1 μm and the inspection time is 8 hours per wafer (20 cm wafer). The electron beam method has a distinctive feature that it is able to inspect for any electrical defects (breaking of wire in the wirings, bad continuity, bad continuity of via). However, the inspection speed thereof is very low, and so the development of an inspection apparatus with higher inspection speed has been expected.
Generally, since an inspection apparatus is expensive and a throughput thereof is rather lower as compared to other processing apparatuses, therefore the inspection apparatus has been used after an important process, for example, after the process of etching, film deposition, CMP (Chemical-mechanical polishing) planarization or the like.
The inspection apparatus of scanning electron microscope (SEM) using an electron beam will now be described. In the inspection apparatus of SEM method, the electron beam is focused to be narrower (the diameter of this beam corresponds to the resolution thereof) and this narrowed beam is used to scan a sample so as to irradiate it linearly. On the other hand, moving a stage in the direction normal to the scanning direction allows an observation region to be irradiated by the electron beam as a plane area. The scanning width of the electron beam is typically some 100 μm. Secondary electrons emitted from the sample by the irradiation of said focused and narrowed electron beam (referred to as a primary electron beam) are detected by a detector (a scintillator plus PMT (i.e., photo multiplier tube) or a detector of semiconductor type (i.e., a PIN diode type) or the like). A coordinate for an irradiated location and an amount of the secondary electrons (signal intensity) are combined and formed into an image, which is stored in a storage or displayed on a CRT (a cathode ray tube). The above description shows the principle of the SEM (scanning electron microscope), and defects in a semiconductor wafer (typically made of Si) in the course of processes may be detected from the image obtained in this method. The inspection speed (corresponding to the throughput) is varied in dependence on an amount of primary electron beam (the current value), a beam diameter, and a speed of response of the detecting system. The beam diameter of 0.1 μm (which may be considered to be equivalent to the resolution), the current value of 100 nA, and the speed of response of the detector of 100 MHz are the currently highest values, and in the case using those values the inspection speed has been evaluated to be about 8 hours for one wafer having the diameter of 20 cm. This inspection rate, which is extremely lower as compared with the case using light (not greater than 1/20), has been a big problem (drawback).
On the other hand, as a method for improving the inspection speed or a drawback of the SEM method, new SEM (multi beam SEM) method using a plurality of electron beams and an apparatus therefor have been disclosed. In this method, though the inspection rate can be improved by a number of the plurality of electron beams, there are other problems that since a plurality of primary electron beam is irradiated from an oblique direction and a plurality of secondary electron beam is taken out along an oblique direction from a sample, the detector receives the secondary electrons emitted from the sample only along the oblique direction, that a shadow emerges on an image, and further that secondary electron signals are mixed together because it is difficult to separate respective secondary electrons coming from the plurality of electron beams respectively.
Conventionally, there has been known an evaluation apparatus in which a primary electron beam emitted from an electron gun is focused to be narrower by a lens system to be irradiated onto a surface of the sample, and then secondary electrons emitted from the sample are detected to evaluate the sample surface such as a line width measurement, inspection for the defects thereon or the like. In this kind of evaluation apparatus, the S/N ratio is required to be higher than a predetermined value (for example, 22 to 70). In the case where thermal field emission electron gun is used, it is required to detect the secondary electrons in a range of 1,000 to 10,000 for each pixel.
For example, assuming a detection efficiency being 10%, 104 to 105 pieces of primary electrons have to be irradiated for each pixel. When converting this value into dose, dose D (Q/cm2) may be represented, assuming the pixel size being 0.1 μm square, as:D=104×1.6×10−19Q/(0.1×10−4)2˜105×1.6×10−19Q/(0.1×10−4)2=16 μc/c m2˜160 μc/c m2
Such dose value as in the range of 16 μc/c m2 to 160 μc/c m2 is a significantly large value for the wafer containing a layer of almost completely finished transistor, and such a dose value may have a negative effect thereon that, for example, a threshold voltage Vth of the transistor may increase.
That is, the conventional evaluation apparatus of semiconductor wafer has to employ large S/N ratio and thus large dose, which means when the dose is increased to irradiate large amount of primary electron beam, the threshold voltage of the transistor on the wafer is increased, eventually resulting in a characteristic of the semiconductor device being damaged during the evaluation of a wafer.
Further, in the prior art, there has been another problem that there may occur a location offset between an image of secondary electron beam obtained by irradiating the primary electron beam onto the sample surface and a reference image prepared beforehand, resulting in a deterioration of accuracy in detecting the defect. This location offset may cause a considerably serious problem when an irradiation area of the primary electron beam has an offset with respect to the wafer, and thereby a part of an inspection pattern drops out of the detecting image of the secondary electron beam, which cannot be dealt with only by a technology for optimizing a matching area within the detecting image. This must be a fatal drawback especially in the inspection for a fine micro pattern.